Rapid recycling of glutamate transporters on the astroglial surface

  1. Piotr Michaluk
  2. Janosch Heller
  3. Dmitri A. Rusakov

  • Curated by eLife

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    Summary: In this study, Michaluk et al. examined the membrane dynamics of the main glial glutamate transporter GLT1 in hippocampal astrocytes, which was previously shown to shape synaptic transmission through regulating extracellular levels of glutamate. Using GLT1 tagged on its surface with a pH-sensitive fluorescent marker, GLT1-SEP, the authors performed (1) fluorescence recovery after photobleaching (FRAP) experiments to assess the basal and activity-dependent dynamics of surface GLT1-SEP and (2) super-resolution dSTORM imaging to determine the relationship between GLT1 and PSD-95, an excitatory synapse marker. A large proportion of surface GLT1-SEP underwent turnover with a surface lifetime of 22 s, whereas a smaller fraction (~25%) remained largely immobile, which was decreased upon increased activity. Notably, the cytoplasmic domain of GLT1-SEP was shown to attenuate the basal turnover of surface GLT1 and to facilitate its proximal localization to synapses; moreover, GLT1 cytoplasmic domain was required for activity-dependent increase in the mobile fraction.

    While previous studies using single molecule tracking have demonstrated a role for the lateral diffusion of GLT1 in controlling the recruitment of GLT1 near active synapses, the present study uses powerful optical approaches and analysis tools to access both the surface lateral mobility and the exchange between surface and intracellular pools of GLT1. Furthermore, characterization of the nanoscale organization of GLT1 relative to synapses and its dependence on the C-terminal domain of GLT1 is presented. Altogether, the results are interesting and valuable, and underscore the importance of the GLT1 C-terminus in the membrane turnover and in the activity-dependent lateral diffusion of the surface GLT1. Nevertheless, some of the conclusions are not strongly supported by the data shown.

    Reviewer #2 opted to reveal their name to the authors in the decision letter after review.

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Abstract

Glutamate uptake by high-affinity astroglial transporters confines excitatory transmission to the synaptic cleft. The efficiency of this mechanism depends on the transporter dynamics in the astrocyte membrane, which remains poorly understood. Here, we visualise the main glial glutamate transporter GLT1 by generating its functional pH-sensitive fluorescent analogue, GLT1-SEP. Combining FRAP-based methods with molecular dissection shows that 70-75% of GLT1-SEP are expressed on the astroglial surface, recycling with a lifetime of only ~22 s. Genetic deletion of the C-terminus accelerates GLT1-SEP membrane turnover by ~60% while disrupting its molecule-resolution surface pattern as revealed by dSTORM. Excitatory activity boosts surface mobility of GLT1-SEP, involving its C-terminus, metabotropic glutamate receptor activation, intracellular Ca 2+ signalling and calcineurin-phosphatase activity, but not the broad-range kinase activity. The results suggest that membrane turnover, rather than than lateral diffusion, is the main ‘redeployment’ route for the immobile fraction (20-30%) of surface-expressed GLT1. This reveals a novel mechanism by which the brain controls extrasynaptic glutamate escape, in health and disease.

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  1. eLife

    Reviewer #3:

    In this study, Michaluk et al. explored the membrane dynamics of the main glial glutamate transporter GLT1 in hippocampal astrocytes, which was previously shown to shape synaptic transmission through regulating extracellular levels of glutamate and whose dysfunction may lead to pathologic conditions. Their results underscore the importance of the GLT1 C-terminus in the membrane turnover as well as in the activity-dependent lateral diffusion of the transporter at the plasma membrane.

    To access GLT1 dynamics, the authors generated and imaged a pH-sensitive fluorescent analogue of the GLT1a isoform, namely GLT1-SEP, which fluoresces when exposed to the extracellular space but not in low pH intracellular compartments. By performing Fluorescent Recovery After Photobleaching (FRAP) in astrocytes from cell cultures, they show that about 75% of GLT1-SEP dwell at the cell membrane with a lifetime of about 22 s. Super-resolution dSTORM imaging further revealed that surface GLT1 distributes in clusters showing a spatial correlation with PSD-95 synaptic marker. In astrocytes from cell cultures or brain slices, the authors were able to monitor lateral diffusion of GLT1-SEP at the plasma membrane with FRAP; they recapitulated previous findings based on single molecule tracking experiments and showed that 25% of surface GLT1-SEP remains immobile (or slowly mobile) and that this immobile fraction decreases upon elevated network activity. Interestingly, deleting the C-terminus of GLT1-SEP does not alter much the intracellular fraction of GLT1-SEP, the fraction of immobile GLT1-SEP at the membrane or its ability to organize in clusters under basal conditions. However, GLT1-SEP lacking the C-terminus show a higher turnover at the membrane under basal conditions and surface GLT1-SEP clusters are not associated with synaptic markers anymore. Finally, removing the GLT1 C-terminus blocks the increase in the mobile fraction that is normally observed upon elevating neuronal activity.

    Strengths:

    While previous studies have unveiled a role for the lateral diffusion of GLT1 in controlling the recruitment of GLT1 near active synapses, the present study uses powerful optical approaches and analysis tools that allow for the monitoring of both lateral mobility and the exchange between membrane and intracellular fractions of GLT1. Furthermore, important and original information is provided about the nanoscale organization of GLT1 transporter at proximity of synapses and the fact that this organization depends on the C-terminal domain of GLT1. The results unveil an important role for membrane turnover as a possible 'redeployment' route for the immobile fraction of GLT1 at the plasma membrane.

    Weaknesses:

    1. Although overexpressed GLT1-SEP displays a similar expression pattern as endogenous GLT1 (assessed through dSTORM experiments), the expression level of GLT1-SEP relative to endogenous GLT1 has not been addressed by the authors. In particular, whether overexpressing GLT1-SEP impacts glutamate uptake currents and whether this could affect membrane turnover has not been measured.

    2. The authors did not test the impact of neuronal activity on membrane turnover or surface distribution of GLT1-SEP, like they did for lateral mobility. This would be important to provide support for the 'redeployment route' hypothesis that the authors propose.

    3. The FRAP data in organotypic slices looking at the effect of deleting GLT1 C-terminus, blocking mGluRs or buffering Ca2+ with BAPTA on GLT1 lateral mobility (Figure 5C-G) is not very convincing. The trend for lower immobile fraction upon 4AP compared to control is maintained across conditions. The lack of statistical difference between control and 4AP in Fig. 5D, 5E and 5F might come from the smaller n number (n = 30-48) compared to the control condition (n = 72) and/or higher variability.

    4. The importance of GLT1 membrane turnover for controlling glutamate spillover ('extrasynaptic glutamate escape) and synaptic transmission/plasticity is missing.

    5. While providing new information about the turnover of the GLT1a isoform, this study does not provide information about other GLT1 isoforms, in particular GLT1b, which contain unique C-terminal domains and which could thus display different membrane and lateral diffusion dynamics. The authors should justify why they focused on this specific isoform.

  2. eLife

    Reviewer #2:

    A state of the art imaging of the dynamics of astroglial glutamate transporters that certainly add novel perspective into this, quite important and hot field. Experiments and clean and convincing, the data obtained fully support conclusions.

    Comments:

    1. The authors mention the importance of efficient glutamate uptake in the development of neuropathological conditions, but do not discuss this in regards to their results. Such a discussion would seem relevant.

    2. Authors conclude that the membrane turnover pathway should be a particularly important GLT-1 resupply mechanism near excitatory synapses as some earlier studies have found the lowest lateral membrane mobility of GLT-1 there. In this context, it would be of interest to have some quantitative tips as to the relationship between the level of excitatory activity and the occupancy of local GLT-1.

    3. There is a recent work implicating the C-terminus in the surface assembly of GLT-1 (Peacy et al Mol Pharm 2020), which seems relevant to the present findings. Please discuss further.

    4. Functional activity of glutamate transporters is linked to (and is being regulated by) astroglial Na signalling; any suggestions how proposed turnover cycle may affect cytosolic Na+ dynamics

    5. The Fig. 5A imaging data seem to nicely provide both surface and cytosol labelling of the same cell. Perhaps the authors could thus assess the distribution of surface-to-volume ratios across live astroglia: to my knowledge, such data has not yet been available.

    6. Figure 1B: Please provide further detail regarding fast-exchange solution application, its physical arrangement, etc.

    7. Figure 2, whole-cell photobleaching: Please expand on what is 'tornado' mode scanning and how it has been applied.

    8. Figure 3, dSTORM data: Please provide further details regarding the numbers of sampled ROIs and/or individual molecules / distances analysed.

  3. eLife

    Reviewer #1:

    Astrocyte glutamate transporter, GLT1, plays a crucial role in confining the levels of extrasynaptic glutamate, and therefore, understanding the cellular basis by which surface dynamics of GLT1 is regulated has implications in regulating glutamatergic transmission. Here, Michaluk et al. perform FRAP experiments using pHluorin (SEP)-tagged GLT1, and present a careful quantitative characterization of GLT1 surface dynamics that takes into account both lateral diffusion and exocytic delivery. The authors report that 25-30% of surface GLT1 represent immobile fraction which may be subject to slower exchange via exocytic delivery from intracellular compartments. In addition, the cytoplasmic domain of GLT1 plays a role in regulating GLT1 subcellular localization patterns and its activity-dependent dynamics. While the roles for mGluR and calcium-signaling mechanisms are explored, given the drugs have been applied under conditions in which neurons are equally affected, whether mGluR and calcium signaling involving calcineurin are engaged in astrocytes to impact GLT1 remains to be established. In addition, the super-resolution imaging, which does not discriminate between surface and intracellular pool of GLT1, is not well connected to the FRAP results, which is performed blind to the location of synapses.

  4. eLife

    Summary: In this study, Michaluk et al. examined the membrane dynamics of the main glial glutamate transporter GLT1 in hippocampal astrocytes, which was previously shown to shape synaptic transmission through regulating extracellular levels of glutamate. Using GLT1 tagged on its surface with a pH-sensitive fluorescent marker, GLT1-SEP, the authors performed (1) fluorescence recovery after photobleaching (FRAP) experiments to assess the basal and activity-dependent dynamics of surface GLT1-SEP and (2) super-resolution dSTORM imaging to determine the relationship between GLT1 and PSD-95, an excitatory synapse marker. A large proportion of surface GLT1-SEP underwent turnover with a surface lifetime of 22 s, whereas a smaller fraction (~25%) remained largely immobile, which was decreased upon increased activity. Notably, the cytoplasmic domain of GLT1-SEP was shown to attenuate the basal turnover of surface GLT1 and to facilitate its proximal localization to synapses; moreover, GLT1 cytoplasmic domain was required for activity-dependent increase in the mobile fraction.

    While previous studies using single molecule tracking have demonstrated a role for the lateral diffusion of GLT1 in controlling the recruitment of GLT1 near active synapses, the present study uses powerful optical approaches and analysis tools to access both the surface lateral mobility and the exchange between surface and intracellular pools of GLT1. Furthermore, characterization of the nanoscale organization of GLT1 relative to synapses and its dependence on the C-terminal domain of GLT1 is presented. Altogether, the results are interesting and valuable, and underscore the importance of the GLT1 C-terminus in the membrane turnover and in the activity-dependent lateral diffusion of the surface GLT1. Nevertheless, some of the conclusions are not strongly supported by the data shown.

    Reviewer #2 opted to reveal their name to the authors in the decision letter after review.

  5. Version published to 10.1101/2020.11.08.373233 on bioRxiv